1. Field of the Invention
The present invention relates to a circuit for driving a motor.
2. Description of the Related Art
A drive shaft of a motor which is used, for example, for raising and lowering a window of a vehicle must rotate in both forward and reverse directions. Conventionally, such a motor is therefore driven, for example, by a drive circuit 100 as shown in FIG. 8, in which four NPN transistors 102, 104, 106 and 108 are connected to a motor 110 so that an H-shaped bridge circuit is formed. The collectors of the transistors 102 and 104 are connected to a power supply terminal 112 while the emitters of the transistors 106 and 108 are connected to a ground terminal 114. Each of the transistors is brought into an ON state by a base current supplied by an unillustrated control circuit. The power supply terminal 112 is connected to the positive terminal of a battery (not shown) while the ground terminal 114 is connected to the negative terminal of the battery.
When the drive shaft of the motor 110 is rotated in a predetermined direction, each of the transistors 102 and 108 of the drive circuit 100 is brought into an ON state. Accordingly, current flows through the motor 110 in a direction indicated by arrow A, via a power supply line and the transistor 102, so that the motor 110 is driven. When the drive shaft of the motor 110 is rotated in the direction opposite to the predetermined direction, each of the transistors 104 and 106 of the drive circuit 100 is brought into an ON state. Accordingly, current flows through the motor 110 in a direction indicated by arrow B, via the power supply line and the transistor 104, so that the motor 110 is driven.
When the ignition switch of a vehicle is turned off, an induced voltage is generated in a coil of an alternator, whereby a reverse voltage of -80 to -100 V (hereinafter referred to as "negative surge" is applied to the drive circuit 100 for an extremely short period of time (for example, a few milliseconds). Accordingly, there is the possibility that transistors of the drive circuit 100 will be damaged by the negative surge. In order to prevent the high voltage from being applied to the transistors even when the negative surge is produced and in order to protect the transistors, diodes 116, 118, 120 and 122 are connected between the emitters and collectors of the transistors 102, 104, 106 and 108, respectively.
Further, in a case in which the rotation of the drive shaft of the motor 110 is hindered under the condition that the motor 110 is being driven by maintaining the ON state of the transistors 102 and 108, and the transistor 102 is subsequently turned off, an induced voltage is generated in the coil of the motor 110. The electric potential at a point P.sub.1 shown in FIG. 8 becomes negative (hereinafter referred to as "counterelectromotive voltage"). The 0N state of the transistor 108 is therefore maintained even after the transistor 102 has been turned off so as to make the current circulate via the diode 120, point P.sub.1 motor 110 and transistor 108, thereby absorbing the counterelectromotive voltage.
Moreover, in the case where the rotation of the drive shaft of the motor 110 is hindered under the condition that the motor 110 is being driven by maintaining the ON state of the transistors 102 and 108, and the transistor 108 is subsequently turned off, an induced voltage is generated in the coil of the motor 110, whereby the electric potential at a point P.sub.2 shown in FIG. 8 becomes higher than the voltage of the power source (counterelectromotive voltage). The 0N state of the transistor 102 is therefore maintained even after the transistor 108 has been turned off so as to make current circulate via the diode 118, transistor 102, point P.sub.1, motor 110 and point P.sub.2, thereby absorbing the counterelectromotive voltage. The locking of the motor occurs, for example, when the movement of a power window glass which is driven by the motor 110 is obstructed by snow.
Moreover, in a case in which the transistor 102 is turned off when the motor 110 is being driven and the transistors 102 and 108 are on, the rotation of the drive shaft of the motor 110 continues due to inertia, whereby the motor 110 generates electrical energy. Due to the generation of the electrical energy, voltage is produced at the point P.sub.1 shown in FIG. 8 (hereinafter referred to as "brake voltage"), so that the rotation of the drive shaft of the motor 110 is not stopped immediately. Therefore, the transistor 106 is turned on at the same time as the transistors 102 and 108 are turned off, so as to make current circulate via the diode 122, point P.sub.2, motor 110 and transistor 106, thereby absorbing the brake voltage. Accordingly, the drive shaft is braked and the rotation is instantaneously stopped.
When electrical wires are connected to the positive and negative terminals of a battery of a vehicle, the electrical wires may be mistakenly connected to the wrong terminals, and the polarity of the voltage applied between the power supply terminal 112 and the ground terminal 114 of the drive circuit 100 is reversed. In such a case, heavy-current flows from the ground terminal 114 to the power supply terminal 112 via the diodes 120 and 116 and the diodes 122 and 118, because the forward direction of the diodes 116, 118, 120 and 122 coincides with the polarity of the applied voltage. This causes a drawback in that an unillustrated fuse for protecting the drive circuit 110 is blown.
To prevent the occurrence of the above-mentioned drawback, there has been proposed an approach in which a diode 124 is connected between the power supply terminal 112 and the collectors of the transistors 102 and 104, as indicated by the broken line in FIG. 8, so that the forward direction of the diode 124 becomes opposite to the direction of the above-mentioned heavy-current. Although this approach makes it possible to prevent the short between the power supply terminal and the ground terminal, the power consumption of the diode 124 increases when the drive current of the motor 110 increases, which causes a drawback in that the quantity of heat generated by the diode 124 increases.
Moreover, in an attempt to obviate this problem, when a diode 126, instead of the diode 124, is connected between the power supply terminal 112 and the ground terminal 114, as indicated by the broken line in FIG. 8, so that the forward direction of the diode 126 coincides with the direction of the above-mentioned heavy-current, the diode 126 consumes no power under ordinary operating conditions and is capable of protecting the drive circuit 110, by allowing a short current to flow through the diode 126, when the electrical wires are conversely connected to the terminals of the battery. However, in such a case, a fuse for protecting the electrical wires is blown.